1. Field of the Invention
The invention concerns a process for controlling the delivery of single screw compressors.
2. Description of the Prior Art
The control of delivery of single screw compressors or expansion machines comprising a screw meshing with at least one pinion wheel, by moving a part of the casing so as to delay the closing of the threads and to simultaneously delay the connection with the high pressure orifice in order to maintain the compression rate, is known, especially from the French patent application 76/25431, corresponding to French Pat. No. 2,321,613.
This arrangement is nevertheless very expensive because it requires manufacturing portions of the casing that must slide at the very place where one wishes to ensure a very good tightness between the screw and the casing in order to avoid leakages, thus requiring a high accuracy.
It is also known from U.S. Pat. Nos. 3,088,658 and 3,874,828 how to provide a twin-screw compressor with a control device, using orifices that are sequentially connected with the casing by means of a turn valve placed in a bore parallel to the axis of the screws. This arrangement is much less expensive but it has the disadvantage of providing a very bad efficiency at very low operating deliveries because it does not permit to vary the dimension of the high pressure orifice and to maintain the compression rate when the delivery diminishes.
It is also known from the U.S. Pat. No. 3,804,564 to vary the delivery of a single screw compressor by providing orifices in the casing around the screw, the outline of such orifices being inscribed within the width of the top of a thread of the screw, but this solution leads to the same disadvantages as those cited in relation to aforesaid U.S. patents.